The present invention is directed toward surface treatment of silicone hydrogel contact lenses. More specifically, the present invention provides an optically clear, hydrophilic coating upon the surface of a silicone hydrogel lens by subjecting the surface of the lens to an oxidative plasma followed by treatment with a dilute aqueous solution comprising a silicate salt, silicic acid, colloidal silicon dioxide, or combinations thereof. The invention is also directed to a buffered, sterile solution containing a soluble silicate that can be used to both treat and store a silicone hydrogel contact lens.
Contact lenses made from silicone-containing materials have been investigated for a number of years. Such materials can generally be subdivided into two major classes, namely hydrogels and non-hydrogels. Non-hydrogels do not absorb appreciable amounts of water, whereas hydrogels can absorb and retain water in an equilibrium state. Regardless of their water content, both non-hydrogel and hydrogel silicone contact lenses tend to have relatively hydrophobic, non-wettable surfaces.
Those skilled in the art have long recognized the need for modifying the surface of such silicone contact lenses so that they are compatible with the eye. It is known that increased hydrophilicity of the contact lens surface improves the wettability of the contact lenses. This in turn is associated with improved wear comfort of contact lenses. Additionally, the surface of the lens can affect the lens""s susceptibility to deposition, particularly protein and lipid deposition from the tear fluid during lens wear. Accumulated deposition can cause eye discomfort or even inflammation. In the case of extended wear lenses, the surface is especially important since extended wear lens must be designed for high standards of comfort over an extended period of time, without requiring daily removal of the lens before sleep. Thus, the regimen for the use of extended wear lenses would not provide a daily period of time for the eye to recover from any discomfort or other possible adverse effects of lens wear
The patent literature has disclosed various surface treatments for rendering the surface of silicone lenses more hydrophilic and more wettable, including changing the chemistry of the surface layer, coating the surface, and compounding the polymer with additives that subsequently diffuse to the surface.
Among chemical surface modification techniques are non-polymeric plasma treatments and corona treatments. This includes etching or the selective destruction of a surface layer. Surface modification techniques also include the introduction of functional groups onto a surface layer, for example the introduction of oxygenated functions (hydroxyls, carboxyls, etc.) at the surface of organic polymeric materials for the purpose of increasing hydrophilicity, thereby promoting increased wettability. Such techniques may employ flame treatments, corona treatments, or plasma treatments. Plasma treatments, also referred to as radio frequency gas discharge (RFGD), have been increasingly studied for the modification of surfaces. The plasma gas of RFGD contains vacuum UV radiation plus many reactive species, such as free radicals and energetic electrons and ions. Depending on the gas or vapor used in the plasma and the process conditions, the effects of non-polymeric or non-depositing plasma treatment include surface etching or ablation, oxidation, the formation of reactive groups, and combinations thereof.
Silicone lenses have been subjected to plasma surface treatment to improve their surface properties, e.g., surfaces have been rendered more hydrophilic, deposit resistant, scratch resistant, etc. Examples of previously disclosed plasma surface treatments include subjecting contact lens surfaces to a plasma comprising an inert gas or oxygen (see, for example, U.S. Pat. Nos. 4,055,378; 4,122,942; and 4,214,014).
Another type of chemical surface modification that has been disclosed in the patent literature involves the introduction of functional groups absent in the parent polymer by the grafting or immobilization of molecules, oligomers, or polymers onto a surface. Grafting or immobilization typically involves, first, the formation of a grafting site which may comprise the formation of a radical by means of chemical reactions, UV irradiation, ionizing radiation, plasma treatment, or the like. The next step is the reaction of the species to be grafted or immobilized with the active site. Surface grafting typically involves the propogation of the reaction to form an anchored chain, wherein competing solution and interfacial reactions occur. Surface crosslinking may occur.
Coating a lens usually involves adhesion of a surface layer onto the substrate being coated. The coated layer can be relatively thick and its physical characteristics can be significantly different than those of the substrate. For coatings that involve high-energy species, for example, evaporation, sputtering, plasma polymerization, the initial stages of the treatment can involve a surface treatment. A carbon coating can be formed by various hydrocarbon monomers (see for example U.S. Pat. No. 4,143,949) or combinations of oxidizing agents and hydrocarbons, e.g. water and ethanol. See, for example, WO 95/04609 and U.S. Pat. No 4,632,844. Sequential plasma surface treatments are known, for example a first treatment with a plasma of an inert gas or oxygen, followed by a hydrocarbon plasma. See, for example, U.S. Pat. No. 5,326,584. U.S. Pat. No. 4,312,575 to Peyman et al. discloses a process for providing a barrier coating on a silicone or polyurethane lens by subjecting the lens to an electrical glow discharge (plasma) process conducted by first subjecting the lens to a hydrocarbon atmosphere followed by subjecting the lens to oxygen during flow discharge. U.S. Pat. No. 4,143,949 discloses depositing an ultrathin coating of a hydrophilic polymer by polymerizing a vapor of a hydrophilic monomer such as hydroxyalkylmethacrylate under electrodeless (corona) gas discharge conditions.
Non-plasma techniques for forming a coating have been disclosed. For example, U.S. Pat. No. 3,814,051 to Lewison discloses vacuum bonding a uniform hydrophilic quartz surface to a contact lens by vaporizing quartz, namely silicon dioxide, within a high vacuum chamber. The coating of contact lenses by dipping, swabbing, spraying or other mechanical means has been disclosed in U.S. Pat. Nos. 3,637,416 and 3,708,416 to Misch et al. The latter patents disclose a chemical process in which a coupling film-forming organic silicon compound, a vinyl trichlorosilane, is applied to a silicone surface, followed by a silica or silica gel deposit formed by contact with a silicon halide such as tetrachlorosilane or with a silicic ester, more particularly a tetraalkoxysilane. Solutions of such compounds can also be applied in a single step to a contact lens by dipping or the like. In U.S. Pat. No. 3,708,225, Misch et al. states that the capabilities of such solutions can be enhanced by incorporating a small amount of colloidal silica, preferably about 1 to 5 percent, whereby the solutions tend to thicken and become easier to apply, further facilitating the buildup of a silica or silica gel deposit.
U.S. Pat. No. 3,350,216 to McVannel et al. discloses rendering a rubber contact lens hydrophilic by dipping the lens into a solution of a titanate having the formula Ti(OR)4 wherein R is an alkyl group containing 2 to 4 carbon atoms.
Although such surface treatments have been disclosed for modifying the surface properties of silicone contact lenses, the results have been problematic or of questionable commercial viability, which has no doubt contributed to the fact that silicone hydrogel contact lens have yet to be commercialized. For example, U.S. Pat. No. 5,080,924 to Kamel et al. states that although exposing the surface of an object to plasma discharge with oxygen is known to enhance the wettability or hydrophilicity of such surface, such treatment is only temporary.
Although the prior art has attempted to show that the surface treatment of contact lenses in the unhydrated state can be accomplished, there has been little or no discussion of the possible effect of subsequent processing or manufacturing steps on the surface treatment of the lens and no teaching or description of the surface properties of a fully processed hydrogel lens manufactured for actual wear. Similarly, there has been little or no published information regarding the performance of coatings for silicone hydrogel or extended wear lenses in the human eye.
Thus, it is desirable to provide a silicone hydrogel contact lens with an optically clear, hydrophilic surface coating that will not only exhibit improved wettability, but which will generally allow the use of a silicone hydrogel contact lens in the human eye, preferably for an extended period of time. In the case of a silicone hydrogel lens for extended wear, it would be highly desirable to provide a contact lens with a surface that is also highly permeable to oxygen and water. Such a surface treated lens would be comfortable to wear in actual use and would allow for the extended wear of the lens without irritation or other adverse effects to the cornea. It would be desirable if such a surface treated lens were a commercially viable product capable of economic manufacture.
The present invention is directed to a silicone hydrogel contact lens having a silicate-containing coating and a method of manufacturing the same, which coating is hydrophilic and resistant to protein and lipid deposition. The invention is also directed to a method of maintaining the silicate-containing coating, formed by plasma treatment on a dry lens, during subsequent processing or manufacturing, including lens hydration and heat sterilization.
In one embodiment of the invention, the method comprises treating the lens during autoclaving, subsequent to plasma treatment, with a silicon-containing aqueous solution comprising a silicate salt, silicic acid, and/or colloidal silicon-dioxide. Treatment can be achieved during lens manufacture by submerging the lens in the surface-protective, silica-containing or silica-producing aqueous solution, preferably during lens hydration, followed by heating at an elevated temperature. (By the term solution is broadly meant true solutions as well as colloidal particles in solution, which colloids may be formed by supersaturated solutions.)
In a preferred embodiment, the lens is packaged in a silicon-containing solution and the final package is autoclaved for sterilization purposes. It has been found that the silicon-containing solution reduces or prevents the delamination of the silicate coating on the lens during autoclaving, protects against coating deterioration during storage, and increases the hydrophilicity or wettability of the lens. A solution according to the present invention can, therefore, be used as a packaging solution for storage of a lens prior to customer use. Since such a solution has been shown safe for use in the eye, so that a lens packaged in the solution may be placed in the eye without rinsing.